Refine wording when a folder with an invalid name could not be created

Summary: This is follow-on for D13805 that improves the wording to be less redundant and a little bit clearer.

Test Plan:
Before:

After:

Reviewers: Dolphin, Frameworks, VDG, tmarshall, elvisangelaccio

Reviewed By: Dolphin, elvisangelaccio

Subscribers: markuss, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14641